1. Field of the Invention
The present invention relates to a drivetrain for a four-wheel drive motor vehicle, and to a method for controlling a drivetrain of said type.
In motor vehicles, four-wheel drive was originally used almost exclusively in off-road vehicles. In recent years, however, even vehicles designed predominantly for road-going use (such as for example passenger vehicles in the form of sedans, station wagons, SUVs) have frequently been equipped with four-wheel drive, specifically in order to increase driving safety, in particular when traction conditions are unfavorable.
In four-wheel drive vehicles, a distinction is generally made between differential-controlled systems and clutch-controlled systems. In differential-controlled systems, the drive torque from the drive unit is split between the front axle and the rear axle by means of a differential. In clutch-controlled systems, generally only one axle is driven, and the other axle is driven on demand. Here, in the simplest case, a hand-operated clutch can be provided, which is actuated from the passenger compartment. Modern four-wheel drive vehicles of said type, however, use automatically actuated clutches (for example Haldex clutches) which engage the second axle when a rotational speed difference between the axles builds up or has built up. Said systems are also referred to as “hang-on” systems.
As regards driving dynamics, the latter can be influenced in differential-controlled systems for example by means of a variable torque distribution. It is in this way possible to set up a generally oversteering or understeering driving behavior.
In differential-controlled systems, it is also known to lock the longitudinal differential by means of a clutch in the event of a lack of traction, so that torque is transmitted to the axle with the higher friction value.
In clutch-controlled systems, there are known systems with hang-on to the rear axle. These are generally front-wheel drive vehicles, with the rear axle being apportioned torque in the event of a lack of traction at the front axle. Conversely, systems are also known in which generally the rear axle is driven and the front axle is embodied as a hang-on axle.
In the present context, a drive unit is to be understood as a unit for providing drive torque. This can be a motor such as for example an internal combustion engine or an electric motor, either alone or in combination with a transmission.
The transmission can be a manual-shift multi-step transmission, an automatic converter assembly, a dual-clutch transmission, an automatic shift transmission, a continuously variable transmission, etc.
A drivetrain is known from U.S. Pat. No. 6,378,677, in which drivetrain the rear wheels can be controlled individually, that is to say independently of one another, by means of respective clutches. The clutches are embodied as electromagnetic clutches whose engagement is controlled according to the rotational speeds of the wheels. When both clutches are open, drive torque is conducted only to the front axle. When the rear axle clutches are actuated, there is a split of power between the left and the right wheels, thereby providing a differential function.
2. Description of the Related Art
A drivetrain for a four-wheel drive vehicle is known from DE 39 00 638 C2, in which drivetrain the front axle is permanently driven, connected to the output of a drive unit either via a longitudinal differential or directly. The drive unit is also connected (via the longitudinal differential or a hang-on clutch) to a differential of the rear axle. In addition, each rear wheel is assigned an individually-controlled friction clutch, which is arranged in parallel, in order to be able to control the rotational speeds of the rear wheels differently. It is thereby possible, for example when cornering, to improve the cornering behavior by virtue of the rear wheel at the outside of the corner being apportioned a higher torque than the wheel at the inside of the corner.
A similar concept for controlling two friction clutches of a rear-wheel drive motor vehicle is known from DE-C 36 35 406.
In addition, a drivetrain concept of said type is known under the “Honda SH-AWD” name.
In said system, the drive torque is distributed between the front axle and the rear axle in the ratio 30:70 to 70:30, specifically by means of a planetary differential. The torque apportioned to the rear axle is supplied to two independently controllable electromagnetic clutches which are connected to the left and to the right rear driveshafts. The torque apportioned to the rear axle can therefore be distributed to the rear wheels in the ratio 0:100 to 100:0.
It is also possible to increase the rotational speed of the rear wheels over that of the front wheels when cornering. Said drivetrain concept is intended to influence driving dynamics, in particular the yaw moment.
Now, it is however the case that, in corners, the mean radius of the front axle is generally larger than the mean radius of the rear axle. The front axle must therefore rotate faster than the rear axle when cornering. This is generally compensated by the longitudinal differential.
In the cases of conventional differential-controlled systems and conventional clutch-controlled (“hang-on”) systems, it is only possible even with the longitudinal clutch locked to provide a rotational speed synchronization between the front and rear axles. If, therefore, in the case of conventional systems of said type, the hang-on clutch is actuated when concerning, then a twisting moment is built up by means of the underlying surface between the front axle and the rear axle. Said twisting moment is superposed on the drive torque and leads to a reduction in the drive torque at the front axle and to an increase in the drive torque at the rear axle. This generally generates understeering driving behavior.
In the SH-system from Honda as stated above, the rotational speed of the rear axle is increased when cornering.
In order to increase the rotational speed at the rear axle, a highly complex planetary differential is required in the rear axle gearing. Overall, the construction of the SH-AWD system is comparatively complex.